Numerous medical procedures involve the extracorporeal treatment/analysis of an individual's blood in which blood is removed from the individual, provided to an external apparatus which performs the desired treatment/analysis, and thereafter supplied back to the individual. Peristaltic pumps are commonly used in various aspects of these medical procedures not only to transfer the blood, but to introduce certain substances into the blood and/or to remove certain components harvested therefrom which are pertinent to the particular treatment/analysis as well.
Peristaltic pumps generally utilize a rotor having a plurality of spaced rollers which exert a force upon a flexible tube positioned between the rollers and an arcuate, stationary raceway (e.g., a cylindrical surface defining at least a portion of a cavity in which the rotor is positioned). This force is of a sufficient magnitude such that when the associated roller is aligned with the raceway, the tube is at least partially compressed, generally occluded. Consequently, as the rotor rotates the "column" of fluid between adjacently-located rollers is forced through the tube by the progressive compression of the tube about the raceway caused by such rollers.
Besides its primary pumping function, a peristaltic pump is often used for providing additionally either one or both of the following functions--a metering function for ensuring a given flow rate of liquid through the tube, and an occluding function for ensuring the interruption of the flow of liquid through the tube when the pump is stopped, in particular when it is pumping from a reservoir situated above the pump. A change in the positional interface of any of the pump's rollers relative to the tube will impair these three functions and, in particular, will affect the volume of fluid provided by the pump per revolution. For instance, if one of the rollers does not compress the tube as much after the desired positional interface is established, the volume of fluid provided by the pump will be reduced.